1. Field of the Invention
The present invention relates to a SONOS memory device and a method of erasing data therefrom. More particularly, the present invention relates to a SONOS memory device in which an erasing speed is improved by using hot holes and a method of erasing data therefrom.
2. Description of the Related Art
FIGS. 1A and 2A illustrate cross-sectional views showing a principle of recording information to and erasing information from a memory cell of a conventional silicon oxide nitride oxide silicon (SONOS) flash EEPROM (Electrically Erasable and Programmable Read Only Memory). FIGS. 1B and 2B are schematic drawings illustrating energy band gaps when recording information to and erasing information from the memory cell of the EEPROM depicted in FIGS. 1A and 2A.
Referring to FIGS. 1A and 2A, the conventional SONOS flash EEPROM includes a p-Si substrate 1, a source electrode 2 and a drain electrode 3 formed in the substrate 1, and a tunnel oxide film 4, a nitride film 5, a blocking oxide film 6 and a poly-Si gate electrode 7 stacked sequentially on the substrate 1. The tunnel oxide film 4, the nitride film 5, and the blocking oxide film 6 in a SONOS flash memory are collectively referred to as an oxide/nitride/oxide (ONO) film.
Referring to FIGS. 1A and 1B, in order to record information in the SONOS flash EEPROM, the p-Si substrate 1 is grounded and a predetermined positive voltage (VG>0) is applied to the gate electrode 7. Then, an electric field is formed between the substrate 1 and the gate electrode 7, and a Fowler-Nordheim (FN) current is generated across the tunnel oxide film 4.
Electrons traveling between the source electrode 2 and the drain electrode 3 are injected into the nitride film 5 by tunneling through an energy barrier of the tunnel oxide film 4. The electrons e that enter the nitride film 5 are blocked by the blocking oxide film 6 and information is recorded in the memory cell by trapping the electrons in the nitride film 5.
Referring to FIGS. 2A and 2B, when erasing information recorded in the SONOS flash EEPROM, an electric field, in an opposite direction to that used when recording information, is formed by applying a predetermined negative voltage (VG<0) to the gate electrode 7 after grounding the substrate 1. Accordingly, the FN current across the tunnel oxide film 4 is formed in the opposite direction to the direction of the FN current when recording. Thus, the information stored in the nitride film 5 is erased by moving the electrons e from the nitride film 5 to the substrate 1 by tunneling through the tunnel oxide film 4 by the FN current.
When erasing data using a FN current, erasing speed can be reduced by electrons e moving from the gate electrode 7 to the blocking oxide film 6. In a NOR flash memory device, electrons e are partially charged using a Channel Hot Electron Injection (CHEI) method for recording data and a Hot Hole Injection (HHI) method is used for erasing data. However, in the case of a NAND flash memory, a FN current is used for recording and erasing data, since not all of the electrons scattered in the nitride film can be erased using the HHI method.
FIG. 3 is a graph illustrating a relationship between recording time and threshold voltage Vth according to a variation of a gate voltage when data is recorded using a FN current in the SONOS flash EEPROM. FIG. 4 is a graph illustrating a relationship between erasing time and threshold voltage Vth according to a variation of a gate voltage when data is erased using a FN current in the SONOS flash EEPROM.
Comparing FIGS. 3 and 4, it can be seen that the erasing time is longer than the recording time at the same threshold voltage Vth. This longer erasing time is assumed to arise from a phenomenon caused by electrons injected from the gate electrode, interfering with the erasing.